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The study has proved controversial with some scientists saying it was ‘significantly flawed’ (Picture: Getty)

Babies born by Caesarean section are 33% more likely to be diagnosed with autism spectrum disorder, according to a study which considered 20 million births.

The findings, which combined 61 studies from 19 countries dating back to 1999, also said children delivered by Caesarean had a 17% higher chance of developing attention deficit/hyperactivity disorder (ADHD).

The authors from Sweden’s Karolinska Institutet and Australia said: ‘Birth by cesarean delivery was significantly associated with autism spectrum disorder and attention-deficit/hyperactivity disorder.’

But the study did not prove planned or emergency Caesarean births directly cause such conditions.

The study’s authors said that factors which lead to an operation, including an older mother or the risk of premature birth, could potentially explain the link.

Scientists not directly involved with the study told The Daily Telegraph that the findings were ‘significantly flawed’.

Dr Pat O’Brien, from the Royal College of Obstetricians and Gynaecologists, told the paper: ‘This systematic review and meta-analysis shows an association between caesarean birth and autism and ADHD, but a number of underlying factors which may have led to the development of these conditions were not accounted for.

‘Therefore, the findings of this paper do not show that caesarean birth leads to autism and ADHD.’

Dr O’Brien backed the study’s authors in calling for further research to explain the link.